1. Field of the Invention
The present invention relates to a control device of a robot for picking out and conveying a workpiece.
2. Description of the Related Art
A known robot system includes a robot used for picking out randomly-placed workpieces with a tool, and conveying a picked out workpiece to a predetermined position. In such a robot system, in the case where a position and posture of a workpiece to be picked out are not precisely recognized, the position and posture of the workpiece are not specified, and therefore, a positional relationship between the workpiece and the tool cannot be determined at the time of holding the workpiece by the tool. More specifically, in the case where a workpiece is held in an unspecified manner or a randomly-placed workpiece is held at a portion which is easy to pick up, a portion of the workpiece held by the tool may be changed every time. In addition, a posture of the workpiece relative to the tool may be changed every time when the workpiece is held by the tool. As well as these cases, in the case where a position of the center of gravity of the workpiece is unknown even when the position and posture of the workpiece are recognized, it may be impossible to ensure a desired positional relationship between the tool and the center of gravity of the workpiece when the workpiece is held.
Therefore, a workpiece may be held at a portion of the workpiece distant from its center of gravity, or in an unstable posture. In these cases, the workpiece is not stably held. Therefore, a portion of the workpiece held by the tool may be relatively moved, changing a holding state of the workpiece during the conveyance. The workpiece may fall during the conveyance, damaging peripheral devices. These may require operation of the robot system to be suspended, or put workers in a dangerous situation.
Since a positional relationship between the tool and the workpiece is not determined, it may be necessary to know a longitudinal direction of the workpiece or the posture of the workpiece. If the workpiece has a plurality of faces that can be held by the tool, it may be impossible to know which face of the workpiece is actually held by the tool. In these cases, an additional process may be required to detect a portion of the workpiece being held, or a posture of the workpiece, by using a visual sensor. Further, since a position of the workpiece relative to the tool is unknown, an additional process may be required to put the workpiece at another site and hold the workpiece again after the posture of the workpiece is detected by the visual sensor.
Furthermore, in the case where there are a plurality of types of workpieces, or there are defective workpieces or unexpectedly different workpieces, or in the case where the workpiece is fragile, an additional inspection process may be required to check the type of the workpiece or a quality of the workpiece. When a workpiece is picked out from randomly-placed workpieces, a large number of workpieces must be picked out. As a result, increased cycle time and increased cost may be required to pick up and convey the workpiece to a certain position.
JP-A-2011-183537 discloses the related art which involves checking a holding state of a workpiece and holding the workpiece again in another posture as necessary. JP-A-5-212690 discloses the related art which involves measuring a workpiece being held, and determining whether or not the workpiece matches a predetermined type. JP-A-5-241626, JP-A-2004-249391 and JP-A-2011-201007 disclose the related art which involves correcting the posture of the workpiece relative to the tool with the aid of a visual sensor. JP-B-3925020 discloses the related art which involves controlling the robot so as to move the workpiece closer to the center of the container when the robot fails to pick out the workpiece. JP-A-2000-263481 discloses the related art which involves suspending the picking out process to retract the robot when an abnormality is detected at the time of picking out a workpiece. JP-A-5-116081, JP-A-2012-40634 and JP-A-7-205075 disclose a method for calculating a weight and a position of center of gravity of a workpiece, and a direction of a gravity force acting on the workpiece for the purpose of compensating an influence of a gravity force acting to a force acting between the workpiece and the tool.
In the known related art described above, if it is determined that a workpiece is unstably held, it is necessary to place the workpiece at another site to check a position and posture of the workpiece by a checking device. Thereafter, based on the checking result, the workpiece is held again in an appropriate posture. However, in some cases, during the process of checking, the workpiece may fall, possibly damaging the checking device. In addition, with use of such a checking device, not only does it take more time to complete the process, but also results in increased cost or requires a large space. Consequently, the increased cycle time and the increased cost cannot be avoided with the checking device. Further the checking device is not designed to obtain the actual weight of the workpiece, the position of its center of gravity, and data of a force while holding the workpiece. Therefore, it is difficult to determine how stable the workpiece is during the conveyance.
In some cases, when it is difficult to determine the type or quality of a workpiece from its appearance, it may be desirable to easily determine the type or quality of the workpiece during the conveyance.
Therefore, there is a need for a robot system, in which estimation of a holding state of the workpiece, determination of the type of the workpiece, and a test for checking a quality of the workpiece can be easily carried out, in order to selectively implement operation as necessary, depending on a particular condition of the workpiece. In this connection, the holding state of the workpiece may include stability of the workpiece held by the tool, the position and posture of the work, or which face of the workpiece is held by the tool.